Corrosion inhibited phosphate solutions



United States Patent 3,342.749 CORROSION INHIBITED PHOSPHATE SOLUTIONS Avrom R. Handleman, Webster Groves, and Robert P.

Langguth, St. Louis, Mo., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed June 2, 1964, Ser. No. 372,102 6 Claims. (Cl. 252-389) This invention relates to improved aqueous solutions of phosphate salts, which solutions are characterized by having a significantly reduced tendency to corrode copper metal and copper containing alloys. More specifically, the present invention relates to corrosion-inhibited ammonium phosphate solutions suitable for prolonged use in handling and storage equipment made of copper and/ or copper alloys and to the compositions useful for manufacturing them.

Aqueous ammonium phosphate solutions have many uses: One of the most valuable recent developments in the field of fire fighting, and more particularly, forest, brush, and grass fire fighting, was that relating to the use of aqueous solutions of ammonium phosphate salts. Since the present invention is particularly valuable when practiced in connection with forest and brush fire fighting, the present discussion will be directed toward this particular end use. However, it should be kept in mind that the invention is generally useful for inhibiting corrosion of copper by ammonium phosphate solutions.

. Ordinarily, the solutions (often thickened with a viscosity modifying agent so that the solutions stick readily to surfaces with which they come into contact) are dropped by airplane such as air-tankers onto brush, trees and dry grass in the path of a fire in order to slow or stop the progress of the fire. The extensive use of such ammonium phosphate solutions for forest fire control has been slowed because of the concern by those in the field over the natural corrosivity of such aqueous ammonium phosphate solutions toward some of the vital parts of airtankers and storage equipment.

Aqueous solutions containing several weight percent of dissolved ammonium phosphate are considered to be corrosive toward copper metal or copper alloys such as brass and bronze and the like, and are corrosive to an extent suificient to cause great concern when the aqueous solutions are to come into physical contact with vital aircraft parts, since failure of the aircraft parts due even to slow corrosion (i.e., more than about 10 mils per year) occurring over a prolonged period of time could result in the loss of the aircraft and crew. Thus, in order to be considered completely acceptable for use in forest fire fighting by the air-drop method, it is necessary that the normal or ordinary corrosivity of aqueous ammonium phosphate solutions, particularly toward copper metals, be significantly reduced.

Consequently, it is an object of the present invention to provide aqueous solutions containing ammonium phosphate salts useful for forest and brush fire fighting, the normal tendency of which to corrode copper and copper-containing alloys is significantly reduced or substantially eliminated.

It is another object of the present invention to provide novel methods of inhibiting the corrosion of copper equipment by aqueous ammonium phosphate solutions.

It is a still further object of the present invention to provide concentrated, particulated solid ammonium phosphate compositions which are useful in preparing aqueous ammonium phosphate solutions characterized by having a significantly reduced tendency to corrode copper metal and copper-containing alloys.

It is still another object of the present invention to provide novel methods of fighting forest and brush fires, which methods do not subject aircraft involved in the 3,342,749 Patented Sept. 19, 1967 practice of such methods to excessive and/or dangerous corrosion of their vital parts made of copper and copper alloys.

It has now been found that the normal or natural tendency of aqueous solutions of ammonium phosphate salts, such as, for example, monoammonium orthophosphate, diammonium orthophosphate and mixtures thereof to corrode copper equipment can be significantly reduced and even practically eliminated by the presence (in solution) of effective amounts of (a) an inorganic water-soluble thiosulfate compound plus (b) a watersoluble Z-mercaptobenzothiazole compound.

The term inorganic water-soluble thiosulfate is intended to include all of those inorganic thiosulfate materials or compounds that are soluble in distilled water at a temperature of 25 C. to the extent of at least about 0.005 weight percent. It includes, for example, the alkali metal and alkaline earth metal iodides such as sodium, potassium, lithium, rubidium, cesium, calcium, magnesium, barium, barylliurn, and strontium, thiosulfates as well as ammonium thiosulfate. Of these, generally sodium, potassium and/or ammonium thiosulfates are preferred. Actually, since the protection from corrosion that can be afforded copper equipment by practicing the present invention apparently results from the thiosulfate (anion) portion of the inorganic thiosulfate compound, the particular source from which the thiosulfate anions are derived in manufacturing the aqueous ammonium phosphate solutions is not at all critical, provided a sufficient amount of thiosulfate anions are provided in the phosphate solutions to effectively decrease the normal corrosivity of the phosphate solutions toward copper and copper-containing alloys.

The term Z-mercaptobenzothiazole inhibitor is intended to'include the compound Z-mercaptobenzothiazole itself as well as all of the salts thereof that are soluble in a 5 weight percent solution of diammonium orthophosphate in distilled water (pH=7.7, temperature=25 C.) to the extent of at least about 0.0005 weight percent. Thus, this term encompasses alkali metal (such as Na, K, Rb, Cs, Li), alkaline earth metal (such as Mg, Ca, Be, Sr), soluble amine salts (such as methylamine, ethylamine and t-butylamine), and many other salts. The foregoing discussion with respect to the relative unimportance (in so far as the successful practice of the present invention is concerned) of the particular source of the Z-mercaptobenzothiazole material, provided it is well dispersed and/ or dissolved in the ammonium phosphate fire-fighting solutions described in detail below, holds true for the Z-mercaptobenzothiazole portion of the inhibitor combination of this invention as well as for the thiosulfate. Although Z-mercaptobenzothiazole is ordinarily considered to be insoluble in most aqueous systems having pHs below about 10, apparently enough of the Z-mercaptobenzothiazoles is soluble in the preferred aqueous ammonium orthophosphate solutions of the present invention to perform well as a copper corrosion inhibitor when used in combination with one or more of the abovedescribed soluble inorganic thiosulfate compounds.

The inhibitor combinations of the present invention perform well even in very dilute ammonium phosphate solutions, since corrosion is often induced or caused by such solutions by a concentration effect resulting from splashing upon copper surfaces and subsequent evaporation of the water from the aqueous solution. However, the ammonium phosphate solutions in which the inhibitor combinations of the present invention perform particularly effectively are those that contain at least about 0.5 weight percent, and up to the level at about which the solutions are saturated therewith, or even more (such as in the form of an aqueous slurry) of dissolved ammonium phosphate salts, including monoammonium dibydrogen orthophosphate and/ or diammonium monohydrogen orthophosphate salts, and mixtures thereof, and also including the diammonium and monoammonium mixed orthophosphate salts that also contain an alkali metal cation, such as monoammonium disodium orthophosphate, monoammonium dipotassium orthophosphate and the like, no matter from What source these materials were derived, or in what form the materals are initially introduced into the aqueous compositions. This includes, for example, solutions of mixtures of ammonium pyrophosphates and ammonium orthophosphates within the prescribed pH ranges, such as those derived from ammoniating and dissolving superphosphoric acid (i.e., phosphoric acids having P contents above about 72.6 weight percent). Generally, it is preferred that the inhibited aqueous phosphate solutions of this invention have a pH above about 7, and preferably between about 7.2 and about 11, but can be even higher without detrimentally effecting the desired corrosion protection afforded by the present invention. This includes solutions containing diammonium orthophosphate as the only ammonium phosphate salt incorporated into the inhibited fire-control compositions of this invention, which compositions have pI-Is above about 7. When diammonium orthophosphate is the major ammonium phosphate salt contained therein, the preferred pH is from about 7.2 to about 10. When mixtures of monoammonium orthophosphate and diammonium orthophosphate are utilized, pHs within these preferred ranges are generally more desirable. However, compositions having pHs somewhat higher or lower than these preferred ranges can also be utilized to advantage in the practice of the present invention.

Although the presence of even very small amounts of the water-soluble inhibitor combination of the present invention in the aqueous ammonium phosphate solutions described above has an improved corrosion-inhibiting effect upon them, as a general rule, aqueous ammonium phosphate compositions containing at least about 0.0075 and preferably from about 0.01 to about 5 weight percent of one or more of the thiosulfate compounds in the dissolved state should be utilized. Generally optimum corrosion-inhibiting effects can be attained by utilizing at least about 0.02 weight percent of the thiosulfate compound in combination with at least about 001 Weight percent of one of the aforementioned Z-mercaptobenzothiazole materials. Similarly, in the practice of this invention, at least about 00005 weight percent of a 2-mercaptobenzothiazole should be present (dissolved and/ or dispersed) in the ammonium phosphate solution, while preferably from about 0.01 to about 2 weight percent of the 2-mercaptobenzothiazole should be present therein. Thus, the weight ratio of ammonium orthophosphate salt(s) to inorganic thiosulfate salt(s) in the compoistions of the present invention ordinarily falls within the range of from about 350021 to about 01:1, and is preferably within the range of from about 250021 to about 1:1. The weight ratio of ammonium orthophosphate salt(s) to 2- mercaptobenzothiazole in the compositions of this invention ordinarily falls Within the range of from about 250021 to about 2.5:1. Upon analysis of the final aqueous firefighting solutions of the present invention it is practically impossible to determine the actual source of the thiosulfate ions and 2-mercaptobenzothiazole material dissolved therein. However, the weight ratio of total ammonium orthophosphate salt(s) to thiosulfate ions in the composition and the weight ratio of thiosulfate ions to 2-mercaptobenzothiazole can readily be determined. Thus, in the preferred aqueous compositions of the present invention, the weight ratio of ammonium orthophosphate to thiosulfate ions is between about 3500:1 and about 1.5: 1, While the weight ratio of thiosulfate ions to Z-mercaptobenzothiazole in the aqueous compositions of this invention is preferably between about 300:1 and about 0.0035: 1.

A surprising feature of the present invention is that the corrosion inhibitor combination (soluble thiosulfate plus Z-mercaptobenzothiazole) acts synergistically in the protection of the copper from corrosion by the ammonium orthophosphate solutions. Thus, when soluble thiosulfate compounds are utilized alone (as the only inhibitor for copper) in a 10 Weight percent aqueous solution of diammonium orthophosphate, for example, the best protection that can be afforded the copper even when very large amounts of thiosulfate is used, is down to about 15 mils per year (as compared with a no inhibitor control corrosion rate of about 1000 mils per year). By comparison, when Z-mercaptobenzothiazole is used alone in the ammonium phosphate solution, use of large amounts of the inhibitor material results in low corrosion rates, but also results in localized pitting; an effect which is considered unacceptable in the art. By using a combination of these two materials it has now been discovered that excellent protection can be afforded copper and copper alloys that are ordinarily corroded by aqueous ammonium orthophosphate solutions, while at the same time, es'sentially no pitting occurs in the protected metal. In addition,- in order to achieve a certain desired degree of cop er protection, smaller amounts of the combination inhibitor of the present invention can be utilized, than when either of the materials alone is utilized. For example, in a 72 hour corrosion test which will be described in detail in Example I, below, the use of 0.025 Weight percent of so: dium thiosulfate resulted in a copper corrosion rate of about 400 mils per year, the use of 0.025 weight percent of Z-mercaptobenzothiazole resulted in a copper corrosion rate of about 480 mils per year, while the use of 0.0125 weight percent of sodium thiosulfate plus 0.0125 weight percent of 2-mercaptobenzothiazole (a total of 0.025%) in accordance with the present invention afforded an excellent corrosion rate level of only 0.8 mil per year.

The corrosion-inhibited ammonium phosphate solutions of the present invention can also contain materials other than the phosphate salts and the thiosulfate and 2-mercaptobenzothiazole compounds without detracting substantially from the benefits that can be obtained by practicing this invention. For example, the solutions can contain minor amounts of water-soluble surfactants; inorganic and organic complexing agents such as the alkali metal tripolyphosphate, pyrophosphates and trimetaphosphates, as well as the higher polyphosphates such as the hexametaphosphates, and also can contain ethylenediamine tetraacetic acid and various alkali metal and ammonium salts thereof and the alkali metal and ammonium nitrilotriacetates; any of the various sticking agents such as carboxymethylcellulose or thickening agents such as algin, citrus pectates, clays such as bentonites and attapulgites, and the like, guar gum and carboxyethylcellulose; other corrosion-inhibiting ingredients such as the water-soluble fiuosilicates for protecting aluminum in accordance with the disclosure in US. Patent Re. 25,394; and inorganic and organic pigments and dyes; as well as many other materials.

The corrosion-inhibited phosphate compositions of this invention can be manufactured via any of a number of methods without any noticeably detrimental effects upon the ultimate performance of the compositions. For example, the thiosulfate and Z-mercaptobenzothiazole compounds can simply be dissolved by intermixing them into or with the otherwise completely formulated aqueous compositions, or at any other stage during the preparation of the fire-control compositions. Via another method, the thiosulfate and/ or the 2-mercaptobenzothiazole compound can first be dissolved in water, and the resulting solution then intermixed subsequently with the ammonium phosphate materials.

Still another process for manufacturing the aqueous corrosion inhibited phosphate compositions described heretofore involves one of the preferred embodiments of the present invention. This preferred embodiment comprises a concentrate mixture of one or more of the ammonium phosphate salts described heretofore with one ents, including a thickenerif one is desired in--the final firecontrol compositions, as well as dyes, pigments, watersoftening agents, and the like, all of which should preferably be present in the concentrate compositions in minor amounts, as compared to the ammonium phosphate salt(s) contained therein. Preferred concentrate compositions contain at least about 30 weight percent, and still more preferably, at least about 50 weight percent, of ammonium orthophosphate material, and minor amounts (less than 50 Weight percent), based on the weight of the concentrate composition, of the combination corrosion inhibitor described hereinbefore, thickener, other corrosion inhibitions, and other desired additives. Some of the advantages of these preferred concentrate compositions can readily be appreciated when it is realized that at most points from which aircraft are loaded for their various air-drops of firecontrol compositions, very little weighing and handling equipment is available that is capable of manufacturing the fire-control compositions from the individual components. Thus, in the absence of the preferred concentrate compositions described above (which concentrate compositions need simply be dissolved in appropriate amount of water in order to be converted into the final valuable fire-control compositions of the present invention), as a practical matter, such corrosion inhibited final fire-control compositions as those described heretofore often could not be utilized.

Typical examples of the preferred concentrate compositions of this invention include: (percentages are in terms of weight percent) Wt. percent Diammonium phosphate 90.0 Guar gum 9.0 Sodium thiosulfate .75 Z-mercaptobenzothiazole .25

Diammonium phosphate 95 Monoammonium phosphate 4 Sodium thiosulfate 0.5 Z-mercaptobenzothiazole 0.5

Diammonium phosphate 90 Carboxymethyl cellulose 9 Potassium thiosulfate 0.5 Z-mercaptobenzothiazole 0.5

Diammonium phosphate 80 Sodium tripolyphosphate 10 Sodium alginate 9 Ammonium thiosulfate 0.8 Z-mercaptobenzothiazole sodium salt 0.2

Diammonium phosphate 60 Attapulgite Clay 39 Sodium thiosulfate 0.8 Z-mercaptobenzothiazole 0.2

Diammonium phosphate 92 Carboxymethyl cellulose 5 Sodium tripolyphosphate 4 Potassium thiosulfate 0.9 Z-mercaptobenzothiazole 0.1

Diammonium phosphate Monoammonium phosphate 3 Sodium alginate 10 Sodium thiosulfate 1.7 Z-mercaptobenzothiazole 0.3

Diammonium phosphate 85 Guar gum 13 Sodium thiosulfate 1 2-mercaptobenzothiazole 1 Diammonium phosphate 60 Attapulgite clay 35 Carboxymethyl cellulose 4 Sodium thiosulfate 0.8 Z-mercaptobenzothiazole 0.2

Diammonium phosphate 45 Triammonium acid pyrophosphate 45 Sodium carboxymethylcellulose 9 Sodium thiosulfate 0.8 Z-mercaptobenzothiazole 0.2

These concentrate compositions are also useful as corrosion-inhibited de-icer compounds, wherein, either alone or in combination with additional corrosion inhibiting materials, they can be utilized in a manner similar to that in which more corrosive, less desirable, sodium chloride, is presently conventionally utilized. For example, these particulated, solid ammonium phosphate-thiosulfate compounds can effectively de-i'ce aircraft runways, sidewalks, roadways and the like when they are simply spread over ice and/ or snow.

The concentrate compositions of this invention can be prepared via any of a number of convenient procedures, including simply blending or mixing together the appropriate amounts of the various dry ingredients. Although the size of the individual particles in these preferred concentrate compositions is not critical in so far as the practice of the present invention is concerned, it is generally preferred that they be of sufficiently small size to pass through a US. Standard 12 mesh screen. For optimum results, at least about 80 weight percent of these particles should be small enough to pass through a US. Standard 20 mesh screen. Although commercial grades of phosphate and thiosulfate salts do not necessarily meet these optimum standards of particle size, they can readily be utilized in the preparation of the compositions of this invention.

In the following examples, which are illustrative of some of the preferred embodiments of the present invention, all parts given are by weight unless otherwise specified.

Example I Into a conventional aluminum storage tank fitted with a fairly efiicient mixer are charged 8.34 parts of water and 1.2 parts of a pre-prepared blend made up of parts of crystalline technical grade diammonium orthophosphate, 9 parts of sodium carboxymethylcellulose and 0.5 part of sodium thiosulfate and 0.5 part of Z-mercaptobenzothiazole. The resulting mixture is then stirred un- 534W DA T =Corrosion in mils per year wherein W=weight los tduring test in milligrams, D specific gravity of metal, A=exposed surface area in square inches, T=time of exposure to solution in hours,

in order to determine the corrosion that has taken place, expressed in term of mils of penetration per year. The corrosion rate or rate of attack on copper by the composition of Example I is thus found to be about 0.8 mils per year.

By comparison, a composition which is prepared in the same way as that in Example I, above, but without either the sodium thiosulfate or the 2-mercaptobenzothiazole, corrodes similar copper coupons at a rate of about 1,000 mils per year.

In a manner similar to that just described, other compositions containing various levels of some of the combination inhibitor compositions described hereinbefore are tested. Results of these tests are given in Table 1, below:

TABLE L-CORROSION INHIBITOR DATA AGAINST COPPER Example No. Corrosion rate (m.p.y.)

Control 940 8.34 parts of water plus 1.2 parts of a pro-pared blend. Numbers correspond to specific preferred concentnate compositions given above.

Each of the compositions tested are excellent forest and brush-fire retardants. In addition to the aforementioned forest, brush and grass fuels, the corrosion inhibited ammonium phosphate compositions of this invention can be dropped on, around or in front of fires burning in practically any cellulosic fuel. For example, they can be dropped or sprayed on wooden or frame structures, or structures that contain cellulosic components close by an actively burning fire in order to protect the coated structure from burning embers, as well as from spontaneous ignition due to excessive heat from the nearby fire.

What is claimed is:

1. An aqueous copper corrosion inhibited ammonium orthophosphate composition having a pH above about 7 and consisting essentially of, in addition to an ammonium orthophosphate dissolved therein in an amount of at least 0.5 weight percent, (a) a water soluble inorganic thiosulfate in an amount of at least about 00075 weight percent and (b) a 2-mercaptobenzothiazole inhibitor in an amount of at least about 0.0005 weight percent.

2. A corrosion-inhibited aqueous ammonium orthophosphate composition as in claim 1, wherein said inorganic thiosulfate is selected from the group consisting of ammonium, alkali metal, and alkaline earth metal inorganic thiosulfates.

3. A copper corrosion-inhibited aqueous ammonium orthophosphate composition having a pH above about 7 and consisting essentially of at least about 0.5 weight percent of an ammonium orthophosphate salt selected from 'the group consisting. of monoammonium dihydrogen orthophosphate, diammonium monohydrogen orthophosphate, and mixtures thereof, from about 0.0075 to about 5 weight percent of a water soluble inorganic thiosulfate, and from about 0.0005 to about 2 weight percent of a 2-mercaptobenzothiazole.

4. A copper corrosion-inhibited aqueous ammonium orthophosphate composition as in claim 3, wherein said ammonium orthophosphate is present in an amount between about 5 and about 25 weight percent, said thiosulfate is present in an amount between about 0.0075 and about 5 weight percent, and said 2-mercaptobenzothiazole is present in an amount between about 0.0005 and about 2 weight percent; said amounts all based upon the total weight of said composition.

5. A composition as in claim 4, wherein the weight ratio of said thiosulfate to said Z-mercaptobenzothiazole in said composition is from about 300:1 to about .0035: 1.

6. A process for decreasing the corrosivity of copper and copper alloys caused by a normally corrosive aqueous solution containing at least one inorganic ammonium 0rthophosphate salt in an amount of at least about 0.5 weight percent, which process comprises effecting the contact of said copper and copper alloys with said solution having dissolved therein (a) a water soluble inorganic thiosulfate salt in an amount of at least about 0.0075 weight percent and (b) a 2-mercaptobenzothiazole in an amount of at least about 0.0005 weight percent.

References Cited UNITED STATES PATENTS Re. 25,394 6/1963 Martinson 71-34 X 1,997,669 4/ 1935 Arcieri 169-2 2,303,399 12/1942 Schwartz 252-387 X 2,617,713 11/1952 Ayers et al 252-192 X 2,901,428 8/ 1959 Schulenberg 252-192 X 2,972,581 2/1961 Johnson et a1 252-389 X 3,238,136 3/1966 'Willard et a1 252-389 X OTHER REFERENCES Chemical Week. vol. 89, No. 14, October 1961, pp. 39-40.

Condensed Chemical Dictionary, 4th ed. (1950), QD 5 C5 1950 Q2, pp. 46, 415.

LEON D. ROSDOL, Primary Examiner.

M. WEINBLATT, Assistant Examiner. 

1. AN AQUEOUS COPPER CORROSION INHIBITED AMMONIUM ORTHOPHOSPHATE COMPOSITION HAVING A PH ABOVE ABOUT 7 AND CONSISTING ESSENTIALLY OF, IN ADDITION TO AN AMMONIUM ORTHOPHOSPHATE DISSOLVED THEREIN IN AN AMOUNT OF AT LEAST 0.5 WEIGHT PERCENT (A) A WATER SOLUBLE INORGANIC THIOSULFATE IN AN AMOUNT OF AT LEAST ABOUT 0.0075 WEIGHT PE 